Polyesters are condensation polymers that are important in the polymer industry. Common polyester resins include poly(ethylene terephthalate), PET, poly(trimethylene terephthalate), PTT, and poly(butylene terephthalate), PBT. Commercial processes for manufacturing polyesters typically include four steps: esterification, precondensation, finishing, and solid-state polycondensation (SSP). The conventional melt-phase polymerization (MPP) process for manufacturing PET chips comprises the first three of these steps.
The esterification step can be performed in two principal ways. Direct esterification of purified terephthalic acid (PTA) with ethylene glycol (EG) and trans-esterification of dimethyl terephthalate (DMi) with EG or similar diols. Other commercially important glycols include 1,3-propanediol, 1,4-butanediol and 1,4-dimethylolcyclohexane. Direct esterification is considerably more rapid than trans-esterification. The direct esterification process also eliminates the need to recover or recycle methanol that is generated as a by-product
U.S. Pat. No. 4,205,157 discloses that long exposure to high temperatures in MPP causes thermal degradation which can form carboxyl and vinyl end groups. Vinyl end groups can deteriorate into acetaldehyde in the final polyester product. Less than one ppm of acetaldehyde is specified for PET bottle resins. Co-monomers such as purified isophthalic acid (PIA) and diethylene glycol (DEG) must be co-polymerized with PET to meet the bottle grade specification. The co-monomers reduce the temperature of melting the solid PET chips again inside the barrel of an injection molding machine and decrease the crystallization rate during the injectior/cooling cycle of the molten PET to give the injection-molded pre-forms. A homo-polymer of PTA and EG shows a peak melting temperature of about 259° C. by differential scanning calorimetry (DSC), while a co-polymer of bottle resins has a peak melting temperature of about 248° C. Lower temperatures to melt the solid PET chips minimize the generation of acetaldehyde and improve the color of bottle pre-forms. A slow crystallization rate is favorable to produce the transparent pre-forms without haze.
The polyester oligomers from the esterification step are subjected to a second step, pre-condensation, in an intermediate reactor to upgrade the intrinsic viscosity. The finishing step in MPP continues to upgrade the molten PET to higher molecular weights appropriate for fiber grades and bottle pre-polymers. During the finishing step, the highly viscous molten PET is continuously stirred with a specially-designed agitator to increase its surface area for effective removal of EG or other by-products by using a very low vacuum or forcing an inert gas through the reaction mixture.
The molten PET from MPP is cooled and then formed into pellets or pastilles as pre-polymers. Clear pellets are made from extruded strands that are rapidly quenched and chopped up to have a maximum dimension of about 2.5 to 3.0 mm. Amorphous PET pellets from MPP typically have an intrinsic viscosity of 0.57 to 0.65 dl/g which is adequate for textile or carpet applications. Opaque pastilles are generated in a modified MPP process without the finishing step, which reduces the MPP reaction time from 3 to 7 hours down to about 2 hours. Partially crystalline pastilles as pre-polymers have, however, a relatively low intrinsic viscosity of 0.18 to 0.25 dl/g which is not sufficient for textile or carpet grades. Pastilles with a relatively low molecular weight are drop formed to have a maximum dimension of about 2.5 to 3.0 mm. Commercial beverage bottles requires the processing by injection molding and stretched blow molding of PET chips with intrinsic viscosity of about 0.72 to 0.85 dl/g. Hence, prepolymers from MPP must be fed to the subsequent SSP to increase the molecular weight of pellets for bottle uses or raise the intrinsic viscosity of pastilles for bottle, textile, or carpet grades.
SSP is a thermal treatment process to upgrade PET to a desired molecular weight that is proportional to intrinsic viscosity. SSP is typically performed in a gravity-driven moving bed reactor. The prepolymers fed to the SSP unit can be completely amorphous pellets or partially crystallized pastilles. However, amorphous PET pellets are only thermally stable and therefore not sticky up to the glass transition temperature of about 80° C. The SSP reaction temperature is, however, above 200° C. Consequently, it is a pre-requisite to partially crystallize the PET pellets after MPP and prior to continuous SSP in a gravity flow reactor. The potential for stickiness of partially crystallized PET pellets shifts from the glass transition temperature toward the onset of melting temperature of about 235° C. Hence, precrystallizer and crystallizer vessels are typically utilized between the MPP and the conventional SSP reactor. The elevation of the entire complex is usually very high because the precrystallizer is stacked above the crystallizer, and the crystallizer is stacked above the SSP reactor.
In a gravity-driven SSP moving bed with extruded pellets, the maximum reaction temperature is constrained by temperature at which the PET chips tend to stick. The normal operating SSP temperature for a copolymer is approximately 210° C., but the normal operating temperature of a homo-polymer can be as high as 220° C. The SSP reaction time can be as high as 12 to 18 hours to bring amorphous pellets of 2.5 to 3.0 mm in diameter and length and over 24 hours to advance semi-crystalline pastilles of 2.5 to 3.0 mm in diameter to the target intrinsic viscosity of 0.72 to 0.85 dl/g.
In an SSP reaction, either ester interchange shown in Formula 1 or esterification shown in Formula 2 occurs: 
Typically, an inert gas stream, typically nitrogen, is purged through the SSP reactor to heat the PET particles and purge the by-products such as water and ethylene glycol from the SSP reactions. Purge of by-products decreases the occurrence of the reverse reaction and drives the equilibrium toward the poly-condensation. The purge gas stream is purified and recirculated to the SSP reactor. The SSP also provides a polymer with low acetaldehyde content. The acetaldehyde by-products are vaporized, diffuse out of the polymer when heated, and purged away by the inert gas stream during SSP. If the preceding MPP is conducted at relatively low temperatures, as explained in U.S. Pat. No. 4,205,157, it is expected to observe a smaller concentration of vinyl end groups and carboxyl end groups. The ester interchange reaction rate of Formula 1 is faster than the esterification reaction rate of Formula 2. B. Duh, Reaction Kinetics for Solid-State Polymerization of Poly(ethylene terephthalate), J. Appl. Poly. Science, Vol. 81, 1748 (2001) studied SSP of PET prepolymers in a batch fluidization bed and suggested a kinetic model with respect to the active end group concentration. The SSP reaction can be considered as a second-order reaction. The publication explains that if the particle sizes are small enough and the purge gas flow rate is high enough, the kinetics of poly-condensation will tend to be reaction-controlled.
U.S. Pat. No. 4,165,420 discloses a rotary spraying congealer to convert the molten PET into beads having an average particle size of 100 to 250 μm. The beads are sufficiently crystallized during cooling after atomization to obviate a crystallizing step prior to SSP. The beads are then polycondensed in a fluidized bed. U.S. Pat. No. 4,205,157 discloses solid-state polycondensation of precrystallized prepolymers in a fluidized bed. U.S. Pat. No. 5,408,035 discloses an SSP unit comprising two unfluidized moving bed reactors in series preceded by a crystallizer and a preheater.
JP 49-18117 B discloses an SSP reactor comprising multiple discrete fluidized chambers. The PET pellets used had a maximum dimension of 4 mm and an initial intrinsic viscosity of 0.70 dl/g. In one embodiment, a partition separates the discrete chambers until opened to allow the contents to enter the succeeding chamber. In this embodiment, it took 6 hours of residence time to improve the intrinsic viscosity from 0.7 to about 0.98 dl/g. In a second embodiment, partitions separate the discrete chambers that are fluidly communicated by an overflow tube with an opening spaced above the floor of the chamber. Hence, fluidized PET pellets had to randomly spill over the top of the overflow tube to proceed to the subsequent reaction chamber. A total mean residence time of five hours was necessary to upgrade the intrinsic viscosity of the pellets in the reactor from 0.70 to above 0.90 dl/g. However, the distribution of intrinsic viscosity was broad, indicating the undesirable residence time distribution of the pellets that occurs in a reactor where chambers are communicated by an overflow tube.
A single fluidized bed has several advantages for continuous SSP of small PET beads. Fluidizing the beads reduces their sticking tendency because the agitated particles are not in contact long enough to stick together. Hence, a precrystallizing step after MPP and before SSP can be obviated. Moreover, fluidization of small PET beads assures rapid diffusion of by-products away from the beads to prevent the reverse reaction from occurring. Hence, bottle-grade polyester can be obtained with less reactor residence time. A smaller SSP reactor is then required. These improvements reduce the capital cost and the elevation of the SSP complex.
It is highly desirable to design a simple, inexpensive SSP process for producing PET particles with a sufficiently high intrinsic viscosity in less reaction time.
Therefore, an object of the invention is to provide an SSP reactor and process that can upgrade the intrinsic viscosity of polyester particles from the MPP having an initially low intrinsic viscosity. A further object of the invention is to treat the polyester from an MPP process that has omitted a finishing step. An even further object of the invention is to provide an SSP reactor and process that can provide polyester product with a sufficiently high average intrinsic viscosity over a short reaction period. Lastly, an object of the invention is to provide an SSP reactor and process that uses no crystallizer equipment before the SSP reactor.